I do not use guages, but then, I'm not racing either. I can tell you that in heavy weather, I don't use any batten tension at all, this moves the pocket as far forward as possible and flattens the sail (high gear). When it's a gentle breeze day I tension the battens as much as I can to move the pocket aft and increase the power (low gear). On days that are in-between I just guess and go somewhere in the middle between tight and loose. Seems to work for me, I'm just not serious enough about it to use draft guages.

You don't need gauges. Set them the same every time. They should be snug enough that they induce a moderate curve along the length of the batten pocket and there should be no wrinkles in the sail fabric.

Adjusting the batten tension for different windspeeds really has no significant effect on draft position or depth. Sail draft is affected primarily by the sail controls (mainsheet & downhaul) and the sail cut. Draft is also affected to a lesser extent by the taper of the battens. The batten tension should simply be set so that the sail fabric is pulled firmly and no wrinkles exist.

Running the battensloose in high wind is just asking to have a batten tip poke through the sail. You would be better off swapping a stiffer batten, but even that would have limited effects.